1. Field of the Invention
The present invention relates to a liquid crystal display device having improved half-tone viewing angle characteristics. More particularly, the present invention relates to a vertical alignment liquid crystal display device with an improved half-tone viewing angle characteristics such as an MVA (Multi-domain Vertical Alignment) mode liquid crystal display device.
2. Description of the Related Art
For liquid crystal displays (LCDs) using active matrices, TN mode liquid crystal display devices have been widely used in which a liquid crystal material having a positive dielectric constant anisotropy is aligned horizontally to the substrate surface, in a manner twisted at an angle of 90° between two substrates opposite to each other. However, this TN mode has a problem of low viewing angle characteristics. Accordingly, various studies are being carried out to improve the viewing angle characteristics.
As an alternative mode, the MVA mode has been developed in which a liquid crystal material having a negative dielectric constant anisotropy is vertically aligned, and the tilting directions of the liquid crystal molecules at voltage application are regulated by protrusions and openings (slits) formed on the substrate surface. It has improved the viewing angle characteristics greatly.
An MVA mode liquid crystal display device is explained, using FIGS. 1A, 1B and 2. FIGS. 1A and 1B are conceptual views showing an MVA mode liquid crystal display device. FIG. 2 is a conceptual view showing alignment directions of liquid crystal molecules in the MVA mode liquid crystal display device.
In an MVA mode liquid crystal display device, liquid crystal molecules having a negative dielectric constant anisotropy are vertically aligned between two glass substrates. A pixel electrode connected to TFTs is formed on one of the glass substrates, and a counter electrode is formed on the other glass substrate. Protrusions 8 are formed respectively on the pixel electrode and counter electrode, in an alternate manner.
When a TFT is OFF, liquid crystal molecules 4 are aligned in a direction vertical to the interface of the substrate 1 as shown in FIG. 1A. When the TFT is ON, an electric field is applied to the liquid crystal and the tilting directions of the liquid crystal molecules 4 are regulated by the configuration of protrusions 8. By this, the liquid crystal molecules 4 are aligned in a plurality of directions in a pixel, as shown in FIG. 1B. For example, when the protrusions 8 are formed as shown in FIG. 2, the liquid crystal molecules are aligned in the directions A, B, C and D, respectively. Since the liquid crystal molecules are aligned in a plurality of directions as shown above in an MVA mode liquid crystal display device when a TFT is in a state of ON, good viewing angle characteristics are realized.
In the above-described MVA mode, it is not that alignment control films control the tilting directions of the liquid crystal molecules, and therefore, it is not necessary to apply an alignment treating process, represented by a rubbing process, which has been almost inevitably necessary in a horizontal alignment mode represented by the TN mode. Accordingly, in view of the processing, problems related to static electricity and debris due to rubbing are avoided, and washing steps after the alignment treating process is unnecessary. Furthermore, in view of the alignment, there are advantages that the problem of display irregularities due to the fluctuation in pretilting is absent, the process is simpler, the production yield is improved, and a low cost is realized.
In addition, as a technology for regulating the alignment of liquid crystal molecules, a liquid crystal display device is also known that has a first alignment control layer and a second alignment control layer formed by polymerizing a polymer precursor, between the substrates and sandwiching a liquid crystal layer (see Japanese Patent No. 3520376, claims).
However, there are some big problems to be solved also in the MVA mode liquid crystal display devices. One of them is that, as shown in FIG. 3, there are a half-tone region in which the transmittance in the transmittance-voltage characteristics (T-V characteristics) in a state of a polar angle of 60° and an angle of direction of 45° to the tilting direction of a liquid crystal molecule is higher than the transmittance in the T-V characteristics in the direction facing the tilting direction of the liquid crystal molecule (that is, a polar angle of 0°), and another half-tone region in which the relation is that the former is lower than the latter, and accordingly, the color tones at the front and the color tones when seen diagonally are sometimes different from each other. It is to be noted that the relationship of the polar angle and the angle of direction relative to the tilting direction of a liquid crystal molecule is schematically shown in the lower part of FIG. 3.